1. Field of the Invention
The present invention relates to a resin film and to a method for producing a resin film. The present invention also relates to a polarizer and to a liquid crystal device produced by the use of the cellulose acetate film.
2. Description of the Related Art
Application to TVs of liquid-crystal display devices is expanding these days, and with the tendency toward big screen panels in the art, image quality enhancement and device cost reduction are being much desired. Further recently, outdoor use of liquid-crystal display devices has become much popular, and the devices are required to satisfy high-level durability in more extreme weather condition than that for the devices that have heretofore been produced under anticipation of indoor use thereof.
It is generally known that the durability of the polarizer in a liquid-crystal display device is enhanced by sandwiching a polarizing element formed by the use of polyvinyl alcohol (PVA) and iodine with polarizer protective films. As the polarizer protective films, various resin films are used such as typically cellulose acylate films, acrylic resin films, etc.; and they are required to be tough and have excellent optical properties. However, conventional polarizer protective films are not satisfactory in point of the polarizing element durability in outdoor severe environments, especially under high-temperature high-humidity condition.
On the other hand, in case where the resin film for use in polarizer protective films is produced according to a solution casting method, inexpensive resins for which the material cost is low must be used for satisfying the recent requirement for price reduction. Accordingly, it is desired to use inexpensive resins (for example, cellulose acylate having a low degree of substitution) heretofore not used in the art from the viewpoint that the releasability thereof from support in casting is poor and the surface condition of the film is therefore worsened in peeling and the optical properties thereof become ununiform, and to improve the releasability thereof to produce more inexpensive resin films.
JP-A 61-243837 describes a method of casting a cellulose acylate film solution that contains at least one release improver selected from (I) an organic phosphoric acid salt having an alkyl, alkenyl or aralkyl group having from 4 to 22 carbon atoms, (II) an organic sulfonic acid salt having an alkyl group having from 4 to 16 carbon atoms, in which the alkyl group links to the sulfonic acid group via an aryleneoxy group or an alkylene group, and (III) an organic polymer sulfonic acid salt having a sulfonic acid group in the recurring unit-having polymer side chain thereof. The patent reference says that use of the release improver satisfying the requirement makes it possible to reduce the release resistance of cellulose acylate having a degree of acetylation of from 56 to 62% (having a total degree of acyl substitution of from 2.5 to 2.95 or so). In Example in the Patent Reference, used are Compound 6 (sodium salt of organic phosphoric acid with two alkyl groups each having 12 carbon atoms linking to the phosphoric acid group therein), Compound 2 (sodium salt of organic sulfonic acid with two alkyl groups each having 9 carbon atoms linking the sulfonic acid group via the aryleneoxy group and the alkylene group therein), and Compound 3 (sodium salt of organic polymer sulfonic acid having a sulfonic acid group in the recurring unit-having polymer side chain thereof), as the release improver for cellulose acylate having a degree of acetylation of 61.9% (having a degree of total acylation of 2.9).
JP-A 2002-1798389 describes a method for casting a cellulose acylate film dope that contains an acid having an acid dissociation index in an aqueous solution of from 1.93 to 4.50 or its alkali metal salt or alkaline earth metal salt. The patent reference describes the use the organic acid or inorganic acid satisfying the requirement as a release agent. In Example in the patent reference, used are citric acid, tartaric acid, Compound 6 described in JP-A 61-243837 (sodium salt of organic phosphoric acid with an alkyl group having 12 carbon atoms and an alkyl group having 10 carbon atoms linking to the phosphoric acid group therein) and Compound 9 (potassium salt of organic sulfonic acid with an alkyl group having 16 carbon atoms linking to the sulfonic acid group via the aryleneoxy group and the alkylene group therein) as the release agent for cellulose acylate having a total degree of acyl substitution of from 2.7 to 3.0.
JP-A 10-316791 describes a polarizer protective film for which are used cellulose acetate flaks that contain an acid having an acid dissociation index in an aqueous solution of from 1.93 to 4.50. The patent reference describes use of such an organic acid or inorganic acid that satisfies the condition as a release agent. In Example in the patent reference, used are citric acid, citric acid salt and sodium acetate as a release agent and calcium acetate and magnesium acetate as a heat-resistant stabilizer for cellulose acylate having a degree of acetylation of from 55.2 to 61.3% (having a total degree of acyl substitution of from 2.5 to 2.9 or so).